1. Field of the Invention
The present invention relates to a refrigerating apparatus which includes a refrigerant circuit constituted by connecting a compressor, a gas cooler, a pressure reducing unit, an evaporator and the like via pipes and in which a natural refrigerant such as carbon dioxide (CO2) is used with a supercritical pressure as the discharge-side pressure of the compressor.
2. Description of the Related Art
Heretofore, a chlorofluorocarbon-based refrigerant has been used in a refrigerating apparatus, but chlorofluorocarbon has a problem such as ozone layer destruction or global warming. Therefore, the use of chlorofluorocarbon has started to be strictly regulated, and the development of a refrigerating apparatus has been advanced in which a natural refrigerant such as CO2 or hydrocarbon is used as a substitute refrigerant.
In particular, CO2 is the natural refrigerant having a small global warming coefficient, and is incombustible and nontoxic unlike hydrocarbon having inflammability or ammonia having toxicity. Therefore, CO2 is expected as the next refrigerant that is eco-friendly and highly safe.
However, CO2 has a critical point of 31.1° C., 7.38 MPa, and hence a very high pressure is required for performing heat exchange accompanied by phase change such as evaporation or condensation in the refrigerating apparatus. Therefore, CO2 compressed in the refrigerating apparatus is brought into a high-temperature high-pressure supercritical state and discharged from a compressor.
It is known that a method of performing inner heat exchange by use of a cascade heat exchanger (an inner heat exchanger) as shown in FIG. 1 is effective in a case where the refrigerant having the above-mentioned characteristics is used in the refrigerating apparatus (see Japanese Patent Application Laid-Open No. 2004-270517). In FIG. 1, CO2 is used as the refrigerant, reference numeral 11 is a two-stage compressor, 12 is a gas cooler, 13 is a cascade heat exchanger, 23 is an expansion valve (a pressure reducing unit) and 15 is an evaporator.
A low-pressure gas refrigerant sucked by the compressor 11 is compressed into a high-temperature high-pressure state by the two-stage compressor 11, and discharged in a supercritical state. The refrigerant discharged in the supercritical state is cooled in the gas cooler 12, and then flows into a high-pressure-side circuit 13-a of the cascade heat exchanger 13.
The refrigerant passed through the high-pressure-side circuit 13-a of the cascade heat exchanger 13 has the pressure reduced by the expansion valve 23, and the refrigerant in the evaporator 15 cools the evaporator 15 and the periphery of the evaporator. The refrigerant passed through the evaporator 15 has a low temperature and low pressure to flow into the low-pressure-side circuit 13-b of the cascade heat exchanger 13.
Here, the high-pressure-side circuit 13-a of the cascade heat exchanger 13 usually has a temperature higher than that of the low-pressure-side circuit 13-b, so that the heat exchange between both the circuits is performed. Therefore, the refrigerant cooled by the gas cooler 12 passes through the high-pressure-side circuit 13-a, and is further cooled, whereby a refrigerating ability in the evaporator 15 improves.
Then, the refrigerant passed through the low-pressure-side circuit 13-b of the cascade heat exchanger 13 is again sucked by the two-stage compressor 11, thereby forming a refrigerant circuit.
However, the refrigerant discharged from the two-stage compressor 11 has very high temperature and pressure. Therefore, when the gas cooler 12, the evaporator 15 and the like have a high temperature, the refrigerant passes through the gas cooler 12 and the high-pressure-side circuit 13-a of the cascade heat exchanger 13. Even after the cooling is performed, the refrigerant sometimes has a gas state.
The amount of heat absorbed in the evaporator 15 by the refrigerant having the gas state and having the pressure reduced by the expansion valve 23 is smaller than that of heat absorbed in the evaporator 15 by a liquid refrigerant having the pressure reduced by the expansion valve 23. Therefore, to effectively perform cooling in the evaporator 15, the low-temperature liquid refrigerant is preferable.
In a case where the refrigerant having the supercritical state when discharged from the compressor is used as a refrigerant, the amount of the refrigerant with which the refrigerating apparatus is to be filled has to be increased to rapidly perform the cooling. However, there occurs a problem that a large amount of liquefied excessive refrigerant is generated in the refrigerating apparatus in a case where the refrigerating apparatus is sufficiently cooled.